Focusing is easy if you use an aperture of at least f / 8: Most things in the scene are pretty sharply focused. However, if you use large apertures such as f / 2.8, f / 1.8 or even f / 1.2, you will miss the focus a lot more. For best results when focusing with large aperture lenses.
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When we talk about focus, we speak of sharpness. Say you are photographing a portrait. Regardless of whether you use f / 1
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Imagine you use an 85mm lens in a full-frame camera when your subject is at a distance of 2.5 meters. At f / 1.8, the depth of field is only nine centimeters, four centimeters before the focus and five behind.
This means that if you focus the person's hand on six inches of her face, your face will look blurry in the final image. You can see this in the shot below: The hands of the subject are in focus, but they are wide enough in front of his face for his eyes to be not.
Imagine switching to f / 16. This time you have an acceptable focus area of 82 centimeters, 35 centimeters in front of the focal point and 48 centimeters behind. This is a much easier target to reach. You can focus on your outstretched arm and still get a good picture.
The iris is just one of the factors that influence the depth of field. The other big one is the focal length. If you switched to a 35mm lens and found yourself at the same distance from your subject, f / 1.8 will give you a depth of field of 54cm and f / 16 a ridiculous length of 72 meters. Therefore, what is considered a large aperture, with telephoto lenses is getting close. For a 200mm f / 5.6 lens, this is certainly a large aperture, but not for a 17mm lens. Follow the advice in this article if you believe that will help.
Note that I used DOFMaster's online calculator for these calculations. It's a great tool, and I'd suggest spending a few minutes fitting in the equipment you normally use to see what depth of field you get.
Right, with that covert look, let's get involved. With large openings, except for you If you use an old gear designed for manual focus, or hold the camera on a tripod, you must use auto focus. You can not concentrate manually. This means you need autofocus to work for you.
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Using a Single Autofocus Point
Each camera has multiple auto focus points. You can choose between all different points, subgroups or a single focus point. I went into detail on the article about the optimal use of the autofocus.
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In general, a group of auto-focus points will strike the best balance in most situations. This gives you some control over where your camera should focus without being too restrictive. However, if you work with a large aperture, you want to be restrictive. With a shallow depth of field, you can focus on a person's nose and eyebrows while their eyes are blurry.
To achieve this, you will get the best results when you use a single-shot autofocus point – or possibly a very small set of points – just above the point where the camera is supposed to focus. For good portraits, this means placing the active autofocus point directly on the eye of the subject.
The only other autofocus option suitable for large f-stops is autofocus detection if your camera supports it. So your camera does the task of placing the single focus point.
Use Continuous Autofocus
Your camera also has three different autofocus modes: still, hybrid, and burst.
Single autofocus works by searching for the focus and then, once it has found it, it remains locked; Ideal for landscapes, but if you have a shallow depth of field and a moving subject, you will miss the focus.
Continuous autofocus keeps track of your subject; You may miss some shots because your camera focuses on the background for a second, but it is more reliable overall. It's the one you should use.
Hybrid autofocus combines simple and continuous autofocus. The problem is that the hybrid autofocus may not adapt to small movements of the subject at low depth of field. For more information, see our article on the various autofocus modes.
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Shoot in Bursts
Even if you use a single autofocus point and a continuous mode, you will still miss a few shots. It's just the reality to work with a really shallow depth of field. The good thing is that you can increase your numbers with burst mode.
Now you do not have to hold down the trigger anymore, as if you were playing Call of Duty. It's just that when you take a picture, you take three or four shots instead of one picture. Even if your subject moves, the autofocus has time to catch up.
The other thing is, if you're shooting continuously, you do not have to worry about keeping your subject quiet. You can encourage them to move, change poses and be active in general. You get better results and take more natural photos. You'll also get more shots in focus.
Modern cameras are very suitable for focusing with lenses with large aperture. You only have to use the autofocus correctly. One last tip is that you read our article about the focus of the back buttons. This professional technique gives you even more control.
Photo credit: Canon